Annotated Literature Review for NCHRP Report 640 (2009)

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80 1.17.8 Structural Design Rogge et al did not discuss structural design. 1.17.9 Limitations Clogging is of concern mainly in the shoulder areas of the roadway because debris collects there. Other authors have suggested placing an impermeable surface dressing to mitigate these problems. Also Rogge et al note that clogging occurs, “…but clogged pavement still allows drainage through the pavement, whereas dense-graded pavements do not.” Winter maintenance issues with porous asphalt are also of great concern, especially winter icing. Winter maintenance is different for porous pavements because of the “…different temperature behavior for porous asphalt, and because of difficulty maintaining a sufficient salt level at the level of contact between tire and pavement.” Other authors have concluded that ice forms quicker on porous asphalt than a dense mix. The Rogge et al research also concluded that the main physical/mechanical distress in porous asphalt is raveling or particle loss. The problem results from cold mix, low compaction, or segregation from the binder. 1.18 “Noise-Reducing Pavements for Urban Roads.” Danish Road Directorate (DRD). Nordic Road & Transport Research. Volume No. 3. 1999. 1.18.1 General This article describes a research project to investigate the use of two-layer porous asphalt for reducing noise. The Danish government has an initiative to reduce the number of dwellings exposed to a noise level of 65 dB(A) by two-thirds (approximately 100,000 dwellings) by 2010. Based upon research in the Netherlands, the Danish have identified the two-layer porous asphalt as potentially the most effective means of achieving this goal. A two-layer porous asphalt pavement system is composed of a bottom layer of porous asphalt with a large aggregate gradation size and a top layer with a small aggregate gradations size (Figure 11). Both layers contain between 20 and 25 percent air voids. The top layer of porous asphalt has a small aggregate size gradation which keeps dirt and debris from clogging the lower layer. The bottom layer of porous asphalt utilizes a larger aggregate size gradation that produces large size air voids and is used to remove any dirt and debris that penetrates through the upper layer.

81 Figure 11: Principle of Two Layer Porous Asphalt Pavements According to Dutch experience, two-layer porous asphalt has good noise-reducing characteristics compared to dense-graded pavements. The reason for this is the structure of the porous asphalt layer, which contains a large number of interconnected voids. Tires rolling on the road result in air pumping since in front of the tire air is forced away and behind the tire the air is sucked out of the pavement. This pumping action generates a high-frequency noise. On porous asphalt, the pumping, and therefore the noise generated, is reduced because the air is instead pumped down into the interconnected voids of the porous layer. Porous asphalt also reduces noise by absorbing some of the noise emitted by vehicles. On roads with dense pavements, the noise emitted towards the pavement is reflected to the surroundings; however, on porous asphalt some of this noise is absorbed by the pavement through the interconnected void structure. The article states that Dutch experience shows noise absorption depends on the thickness of the porous asphalt layer. The thicker the layer of porous asphalt, the lower is the frequency at which the maximum absorption occurs. 1.18.2 Benefits of Permeable Asphalt Mixtures Benefits listed in the article included reduced potential for hydroplaning, reduced splash and spray, improved visibility of pavement markings and reduced tire/pavement noise levels. 1.18.3 Materials and Mix Design The only information related to materials and mix design included in the article was the aggregate sizes of the upper and lower layers of porous asphalt. Aggregate gradation sizes of 2 to 5 mm and 5 to 8 mm have been used for the top layer. For the bottom layers, sizes of 11 to 16 mm and 16 to 22 mm have been used. 1.18.4 Construction Practices Porous asphalt should be placed over an impermeable layer so that water cannot penetrate into the underlying pavement layer. On roadways with curb and gutter, it is necessary to construct drainage along the side of the road, so that water from the pavement can be led away into the stormwater collection system.